This report describes an instrument for monitoring conditions associated with pavement deterioration that was designed and constructed as part of the Strategic Highway Research Program. The conditions measured are voids or loss of support under a rigid pavement; moisture infiltrating asphalt concrete pavement; fine cracking in pavements; delamination of overlays; and aging of asphalt. These conditions of deterioration are determined by estimating Young's and shear moduli in the pavement, base, and subgrade from the wave propagation measurements. The details of the instrument are described. This report includes a user's manual along with the technical specifications for the device.